1. Field of the Invention
The present invention relates to a plasma processing apparatus and, more specifically, a plasma processing apparatus for processing a base using a plasma as an excitation source, which is suitably used for forming a crystalline or nonmonocrystalline functional deposition film, an insulating film, and a metal wiring film available for a semiconductor device and an optical element, i.e., an electrophotographic photosensitive device, an image input line sensor, an image pickup device, a photovoltaic device, and the like, and used for etching, and in which a high frequency of 20 MHz to 450 MHz can be suitably used.
2. Related Background Art
Various types of plasma processing apparatuses are used for semiconductors and the like in accordance with different application purposes. For example, a variety of techniques have been used to enhance their features in, e.g., film formation of an oxide film, a nitride film, and an amorphous silicon-based semiconductor film using a plasma CVD apparatus and method, formation of a metal wiring layer using a sputtering apparatus and method, and micropatterning techniques using an etching apparatus and method.
Strong demand has recently arisen for improving the film quality and processing ability in a plasma processing apparatus, and various implementations therefor have been examined.
Particularly, a plasma process with an RF power is used because it has advantages in stable electric discharge and thus formation of an insulating material such as an oxide film or a nitride film.
The oscillation frequency of a discharge RF power supply used for a conventional plasma process such as a plasma CVD process is generally 13.56 MHz.
FIG. 1 shows an example of a general plasma CVD apparatus conventionally used in deposition film formation. The plasma CVD apparatus shown in a schematic sectional view for an arrangement of FIG. 1 is a film forming apparatus suitably used when an amorphous silicon film (to be referred to as an a-Si film! hereinafter) is to be formed on a cylindrical electrophotographic photosensitive base. A method of forming an a-Si film using this apparatus will be described below.
A reaction vessel 201 capable of pressure reduction is cylindrical. Part of the reaction vessel forming member also constitutes a second electrode 206. A first electrode 202 which serves as a target film formation base (electrophotographic photosensitive base) as a counter electrode is arranged in the reaction vessel 201. Auxiliary bases 207 and 208 are attached to the top and bottom of the first electrode 202 main body to constitute part of the first electrode. In order to improve uniformity in film thickness and film characteristics, the longitudinal size of the second electrode 206 in the axial direction of a cylinder is set equal to the total length of the first electrode 202 and the auxiliary bases 207 and 208 in the axial direction of the cylinder. The interior of the first electrode 202 is heated by an internal heater 203. An RF power supply 212 is connected to only one portion of the second electrode 206 via a matching circuit 211. An evacuation port 205, a main valve 204, a source gas introduction valve 210, and a source gas introduction port 209 are provided.
The target film formation base 202 serving as the first electrode is set in the reaction vessel (deposition chamber) 201. The main valve 204 is opened, and the interior of the deposition chamber 201 is evacuated via the exhaust port 105 once. Then, the source gas introduction valve 210 is opened, and an inert gas is introduced to adjust a flow rate so as to obtain a predetermined pressure. The heater 203 is energized to heat the target film formation base to a desired temperature of 100.degree. C. to 400.degree. C.
Thereafter, a film formation source gas such as a silane gas, a disilane gas, a methane gas, or an ethane gas, and further a doping gas such as a diborane gas or a phosphine gas, as needed, are introduced via the source gas introduction valve 210, and an exhaust speed is adjusted to keep the interior of the deposition chamber 201 at several 10 mTorr to several Torr.
An RF power of 13.56 MHz is supplied from the RF power supply 212 to one portion of the second electrode 206 via the matching circuit 211 to generate plasma discharge between the second electrode 206 and the first electrode 202. The source gas is decomposed to deposit an a-Si film on the target film formation base 203 serving as the first electrode. Meanwhile, the first electrode is heated to a desired temperature within a range of about 100.degree. C. to 400.degree. C. by the heater 203.
As needed, the target film formation base is rotated by a rotation mechanism (not shown) to improve the distribution of a film thickness in the circumferential direction.
According to this film forming method, a deposition speed for obtaining an a-Si film to satisfactorily perform as an electrophotographic photosensitive body is set at about 0.5 to 6 .mu.m/h. If the deposition speed is further increased, sufficient characteristics for an electrophotographic photosensitive body may not be obtained. When an a-Si film is utilized as a general electrophotographic photosensitive body, a film thickness of at least 20 to 30 .mu.m is required to obtain a good charging ability. The manufacture of the electrophotographic photosensitive body requires a long period of time. For this reason, strong demand has arisen for a technique of shortening the manufacturing time without degrading the characteristics of a photosensitive body.
In recent years, a plasma CVD method using an RF power supply of 20 MHz or more in a parallel-plate plasma CVD apparatus has been reported (Plasma Chemistry and Plasma Processing, Vol. 7, No. 3, (1987) PP. 267-273). This report suggests that the deposition speed can be increased by increasing a discharge frequency to more than conventional 13.56 MHz without degrading the performance of a deposition film, which has received a great deal of attention. A sputtering method and the like using an increased discharge frequency are also reported. Recently, superiority of the increase in discharge frequency over other factors has been widely examined.
In order to increase the deposition speed, a power source having a discharge frequency higher than the conventional discharge frequency of 13.56 MHz is used, and a film is formed in accordance with the same film forming sequence as the conventional one. In this film forming operation, it is confirmed that a film can be formed at a deposition speed higher than the conventional one. In this case, however, another problem which is not significant at a discharge frequency of 13.56 MHz is posed.
More specifically, if a film is formed while rotating a target film formation base, a film having almost the same characteristics as those of a conventional film is certainly deposited. However, sufficient matching of a high frequency power may not be established if a rotation mechanism for a target film formation base is omitted to decrease cost of a film forming furnace and reducing cumbersome maintenance. Further, the length of a wiring for supplying an RF power from a matching circuit to a second electrode is changed to improve the operability of an apparatus. For this reason, electric discharge becomes unstable, or it may not be generated in some cases, varying the film quality in the circumferential direction. That is, when a target film formation base is not rotated, electric discharge becomes unstable and a plasma state in the film forming furnace is greatly localized in the circumferential direction, so that the film quality varies depending on positions.
In addition, the characteristics of a portion having high film quality in the circumferential direction is superior to those of a photosensitive drum on which a film is formed while rotating a target film formation base. In contrast, the characteristics of a portion having poor film quality in the circumferential direction is inferior to those of the photosensitive drum on which a film is formed while rotating the target film formation base. In other words, it is estimated that the photosensitive drum on which a film is formed while rotating the target film formation base is in a state in which a film having good characteristics and a film having poor characteristics are stacked, obtaining average characteristics as a result.
As described above, in film formation using a high frequency of 20 MHz to 450 MHz, if a target film formation base is stationary and a distance between the second electrode and the matching circuit is increased, a phenomenon in which electric discharge becomes unstable or it may not be generated in some cases occurs, resulting in uneven film characteristics. In this manner, undesirable unevenness of an image may occur on a relatively large target object such as an electrophotographic photosensitive body.
Such unevenness of film characteristics poses serious problems when a crystalline or nonmonocrystalline functional deposited film to be used an image input line sensor, an image pickup device, a photovoltaic device, and the like is to be formed, in addition to formation of an electrophotographic photosensitive device. In other plasma processes such as dry etching and sputtering, when having a discharge frequency of 20 MHz to 450 MHz is used, the same unevenness occurs. This unevenness is considered to cause serious problems in practical applications.
In this manner, it is found that, if a connecting wiring distance between the matching circuit 211 for matching a high frequency power source and the cathode electrode 206 is increased, the inductance of the wiring itself is increased, so that sufficient matching may not be established. For this reason, electric discharge becomes unstable, or it may not be generated in some cases. Since the degree of freedom of the wiring between the matching circuit and the cathode electrode is extremely low, an apparatus arrangement having a high degree of freedom of wiring and the like has been desired.